The present invention relates to hollow plastic containers and, more particularly, to blow molded plastic containers with improved self supporting bases.
With the wide acceptance of plastic bottles for packaging carbonated beverages, producers of such containers have developed bottles having self supporting bases. Such a container must be able to withstand the internal pressure necessary to maintain the desired carbonation. Such a pressure is on the order of 75 psi.
One such bottle, with a self supporting base, is disclosed in U.S. Pat. No. 4,261,948, which has become known as the "Supa" bottle. This patent discloses a method of increasing the wall thickness of the bottom structure of the container. This is accomplished by incorporating into the injection molded preform a plurality of longitudinal ribs on the inner surface of the preform near its closed end. When blow molded into the final container shape, these longitudinal ribs form radially outward directed ribs on the inner surface of the container base. The ribs, in addition to adding material to the base, also act to reduce the stretching of the preform and thinning of the wall in the bottom portion, thus providing a base with a thicker base structure than identical containers without a ribbed preform.
The "Supa" preform, however, is only useful in blow molding bottles having a champagne type bottom with an inward depression or a cone in the bottom of the bottle. It is not useful for bottles with a radially ribbed or footed base structure. In addition, the reheat processing parameters for the "Supa" preform must be carefully controlled within a narrow range in order to obtain the stretching and bending of the preform wall necessary to form a supporting base on the bottle. As a result of this narrow range, the scrap rate with the "Supa" preforms can be high.
Accordingly it is an object of this invention to provide a preform which can be used to increase the base wall thickness in a self supporting bottle and which can be used with a variety of bottle base configurations such as footed or hollow radial ribbed bases as well as champagne bottle bases.
It is another object of this invention to provide a preform which can be molded with less restrictive reheat processing parameters.
It is a feature of this invention to provide a preform having a plurality of concentric annular reinforcing ribs on the inner surface of the preform near its base which when blow molded forms substantially concentric annular reinforcing ribs on the inner surface of the bottle base. This feature results in a base of decreasing wall thickness in a radially outward direction which is advantageous because it maximizes plastic in the center of the base where it is required for strength and decreases the thickness of the preform in the area which requires the most stretching and bending to form the feet or legs in the base.
It is an advantage of this invention that the annular reinforcing ribs can network with other reinforcing ribs in the base structure to further increase the base strength.